Dextral Reverse Strike Slip Assemblage

Strike slip displacement within the area comprises a series of anastomising faults trending on average 256°, with near vertical dip angles (Figs. 2-2 and 2-3). Clearly defined slickensides indicate a shallow dextral-reverse sense o f movement, based on slicken-crysts, of 076°/20°. It is important to remember that slickensides record only the latest sense of movement on a fracture surface. A major feature of the fault complex within this area is the large quantity of euhedral (vuggy) calcite precipitated within the fault planes, accommodated by the characteristically undulatory nature of the opposing fault surfaces Figs. 2-11 and 2-12). Where the strike slip assemblage intersects existing B Fault branches two outcomes have been noted (see Fig. 2-2 and 2-3):

i. Where the B Fault branch is favourably orientated, transverse movements are accommodated along the B Fault branch.

ii. Where the B Fault branch is not favourably orientated the B Fault branch is truncated and extensional movements (trend and plunge o f slickensides =

179°/44°), along the B Fault branch are preserved,

2.4.3.3 Jointing

Jointing exposed within the 1330 1-3 Lens development follows similar trends as seen elsewhere within the deposit and are essentially northwest trending fractures infilled with carbonate (see Ashton 1995). Occasional joint faces show deposition of euhedral ‘honeyblend’ sphalerite and very rarely chalcopyrite (Fig. 2-13).

2.5 Interpretation

Determination of the nature of the relationships between the several generations of fractures and the differing styles of mineralisation was the prime purpose of this investigation. These are:

1. There are two distinct fracture sets containing fine-grained sulfides. One set sulfide rich (northwest to northeast trending). The other, carbonate rich (east- northeast trending) parallels the B Fault (Fig. 2-5).

2. No movement along mineralised fractures was observed implying that they are largely dilational.

3. The carbonate-rich ENE extension veins crosscut massive mineralisation in the immediate footwall o f the B Fault (Fig. 2-5).

4. In places, sulfide vein mineralisation crosscuts the carbonate-rich ENE extensional veins (Fig. 2-7).

5. Massive mineralisation is built up within the footwall of the B Fault complex.

6. There is no mineralisation in the hanging wall lithologies of the B Fault, which are direct correlatives of the U Lens host lithologies.

7. At no point is fine-grained mineralisation seen occurring within the B Fault or any of its branches.

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8. Where mineralisation does occur along B Fault related branch fault planes it is coarse-grained euhedral ‘honey’ blende sphalerite.

9. Mineralisation associated with the dextral-reverse strike slip faulting is predominately carbonate rich with only minor coarse-grained euhedral sphalerite (Fig 2-14).

10. The B Fault branches and mineralisation are truncated and displaced by the dextral-reverse strike-slip fault assemblage.

These observations are interpreted below.

1. The existence of NE to NW trending, sulfide-rich, veins within the Navan deposit has been recorded elsewhere (see Andrew and Ashton, 1985), and therefore this study confirms their association with the distribution of high-grade mineralisation. The co-incidence of ENE trending carbonate-rich, sulfide-bearing veins, that parallel the B Fault, points to an overall evolution of tectonic stresses during the mineralising process, from roughly EW extension to SE-NW extension, which became the dominant stress-field during the development of the B Fault.

2. That these NW-NE and ENE mineralised fractures demonstrate only dilational characteristics implies that they did not form during the B Fault extensional phase.

Instead, they must either pre-date, or post-date the B Fault. This, in turn, suggests that;

a) The mineralised fractures formed pre-late Chadian or, b) Post-early Arundian.

However,

3. The observation that carbonate infilled extensional fractures associated with the B Fault crosscut both sulfide-bearing veins, and massive mineralisation strongly suggests that the initiation of the mineralising event pre-dates the latest normal movements on the B Fault, and therefore must be pre-late Chadian in age.

4. That, in places, sulfide-bearing veins crosscut carbonate infilled extensional fractures associated with B Fault indicates that mineralisation continued throughout the evolution of the B Fault.

5. The observation that mineralisation is built-up in the immediate footwall of the B Fault complex suggests that either;

a) The B Fault acted as a seal to migrating metal-bearing fluids or,

b) The B Fault crosscuts a pre-existing mineralised trend.

6. That there is no mineralisation in the hanging wall lithologies of the B Fault, which are direct correlatives o f the U Lens host lithologies, implies again, that either;

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a) The B Fault acted as a seal to metal-bearing fluids or,

b) The B Fault crosscuts a pre-existing mineralised trend.

Although at this locality the B Fault plane is infilled by a thick shaley gouge, this is not typical. More commonly, the B Fault comprises an open cavernous structure containing calcite +/- barite +/- minor pyrite (Fig. 2-14). O f course, it may be that this apparent permeability is the result of fluids moving through the structure during late Carboniferous tectonism. The part of this study dealing with metal distributions in the Navan orebody (Chapter 3) demonstrates the existence of strong NE mineralised trends within the deposit similar to those noted by Andrew and Ashton (1985). These trends are associated with NNE, NE and ENE, steeply dipping normal faults that are, in places, truncated by the major extensional faults (e.g. B Fault) implying that they pre-date the main extensional event. This evidence is in concordance with points 3 and 4 above, and therefore the displacement of a pre-existing mineralised trend as an explanation for the observed features is preferred.

7. That no fine-grained mineralisation is found within the plane o f the B Fault, or any of its branches is a common feature of the orebody. However, this does not imply that it was never there, only that this absence may be the result of fluids capable of sulfide dissolution moving through the structure during a later event.

8. The existence of coarse-grained, euhedral 'honey-blend' sphalerite along some of the B Fault branches suggests that at times metal-bearing fluids did access these fractures (Fig. 2-14).

9. The vuggy carbonate and coarse-grained euhedral sphalerite seen within the dextral-reverse, strike slip assemblage again implies metal-bearing fluids had access to this structure.

10. The observation that the dextral-reverse, strike-slip assemblage truncates and displaces all Lower Carboniferous structures suggests that it post-dates the formation o f the B Fault, and is therefore younger that early Arundian. When coupled with the presence of other dextral-reverse, strike-slip faulting within and around the orebody, final movements of which are thought to have occurred during the Upper Carboniferous (see Phillips and Sevastopulo, 1986; Ashton

1995; Ashton et al., 1992), I suggest that this structure is related to that event.

The occurrence of untypical euhedral ‘honeyblend’ sphalerite within the B Fault branches and within the dextral-oblique strike-slip assemblage, suggests that at certain times sulfides have been allowed to precipitate under conditions dissimilar to those prevalent during the economic mineralisation event. Textures exhibited by economic stage mineralisation indicate that the environment of deposition was far from chemical equilibrium both with respect to fluid mixing and wall-rock reactions (Anderson et al., 1998). For example, skeletal and dendritic crystal forms, stalactitic morphologies, coupled with the fine-grained nature o f the mineralisation indicating rapid precipitation. A far more stable environment or

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one closer to chemical equilibrium must have existed during the later event. The presence of euhedral ‘honey-blend’ sphalerite in association with B Fault branches suggests that fluids moving along these fractures had come into contact with sulfides along their flow path, and re-precipitated dissolved base metal species as sulfides within favourable locations.

A sequence of events can be built up that covers the structural evolution of this part of the ore body in connection with the economic stage, and subsequent mineralisation events.

1. Development of northwest and northeast trending fracture sets.

2. Coeval or later economic mineralisation.

3. Development of east-northeast trending fractures during continued mineralising activity (possibly precursor fractures to the B Fault proper).

4. Development of the B Fault complex with continuing mineralisation and possible late mobilisation of sulfide phases causing precipitation o f coarse-grained euhedral sphalerite on the B Fault branches.

5. Development of the dextral reverse strike slip fault assemblage and jointing, again with minor mobilisation and precipitation of sulfides.

2.6 Conclusions.

This study elucidates the age of mineralisation in relation to timable events during the evolution of the Navan deposit. Access to a drift located in the hanging wall of 1-3 Lens has allowed the mapping of six generations of mineralised and unmineralised fractures. The temporal relationship o f these structures indicates that three fracture sets, northeast, northwest, and east-west trending, existed prior to, or during the mineralising event. These fractures are purely dilatational in character. Carbonate-infilled extensional veins associated with the B Fault (one of a series of major, partly listric, extensional faults that control the current disposition of the orebody) cut across the mineralised NE, NW and E-W fractures, as well as massive and disseminated ore. Conversely, this mineralisation crosscuts the carbonate-infilled extensional veins elsewhere within the same exposure.

Northwest trending joint sets as well as an east-west trending sub-vertical, dextral- reverse strike-slip fault complex, post-date all the above structures, and are interpreted as the result of Upper Carboniferous tectonism. Other than the occurrence of light coloured, coarse-grained, euhedral ‘honey-blend’ sphalerite along fracture surfaces, no sulfide mineralisation is associated with those putative Upper Carboniferous structures at this particular exposure. These observations force the conclusion that mineralisation within this part of the Navan deposit was concurrent with the initial development of the B Fault, and continued during the evolution of that major structure through to the late Chadian.

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LISCARTON FAULT

F26 RANDALSTOWN

FAULT / F24

260m

CASTLE FAULT

F26

A-C FAULT COMPLEX

B FAULT PLANE

T FAULT PLANE

Area of study covered by Chapter 2

YFAULT

BFAULT M FAULT

TFAULT

Figure 2-1 Structural plan of the Navan deposit showing the location of the study area covered by Chapter 2.

Structural plan of the 133013LHWA and associated crosscuts.

1330 13LHXR (Fig. 2-4) 20 metres

1330 13LHWA